Method of examining the state of crystallization of calcined aluminum oxide



Patented Oct. 20, 1936 UNITED STATES PATENT OFFICE METHOD OF EXAMININGTHE STATE OF CRYSTALLIZATION OF CALCINED ALUMI- NUM OXIDE of Germany NoDrawing. Application January 3, 1933, Serial No. 650,051. In GermanyNovember 18, 1932 2 Claims.

My invention relates to a method of examining the state ofcrystallization of calcined nonpla'stic metallic oxides.

It has been found in some cases when manu- 5 facturing shaped bodies ofnon-plastic metallic oxides according to the method of molding by usinga casting slip activated with acid that the removal of the moisturecould not be accomplished in a uniform manner. The method hithertoemployed has the drawback that the mass hardens too rapidly with theresult that cavities are formed in the bodies, causing collapse onremoving them from the mold or difiiculties incident to the removal.Cracks and fissures may form particularly in cases requiring a mold witha core. Flaws and faults may occur during subsequent baking even in thecase of bodies which are removed from the mold without apparent trouble.Such bodies contain flaws or contract as a result of an irregularshrinkage or they are not completely compact.

The reason for this behavior is the following: The calcined metallicoxides on the market are not always quite uniform. If the material, forinstance, aluminum oxide is roasted or fired at a low temperature sothat it consists substantially of gamma corundum, the material isequally unsuitable for the method of molding as if it were fired at ahigh temperature and would consist of alpha corundum. Tests have shownthat the best results may be obtained, if the material is in atransition state, i. e., passes over from gamma corundum into alphacorundum.

Tests carried out by the microscopic method to investigate this statehave not been successful; not even with the aid of Rontgen diagrams orby determinating the losses due tofiring or the hygroscopicity, thespecific weight or the weight by volume.

According to the invention, the property inherent in non-plasticmetallic oxides to form colored bodies with the aid of organic pigments.The oxides absorb the pigment in a more or less 45 different mannerdepending upon their state of crystallization so that the state ofcrystallization may be determined by the intensity of the coloring.

In the case of alumina a treatment with aliza- 50 rin red has beenproposed. In this case a madder lacquer known to those skilled in theart of dyeing is formed. To this end, an aqueous solution of alizarinred is prepared by solving, for instance, 2 grams of alizarin red in 1litre of water. in a portion of this solution which is then stirred forabout half an hour. The coloring takes place during this time. After thesolid parts have been withdrawn from the solution and dried either acomplete coloring of all particles or a partial col- 10 oring of theindividual particles to a greater or lesser extent may be observed withthe aid of a microscope depending upon the kind of material. Since ithas been further found that the oxides calcined at high temperatures donot color, 15 as for instance, alpha corundum in the case of alumina,whereas thematerial fired at a low temperature assumes an intense colorwhich corresponds to gamma corundum in the case of alumina, this methodof coloring constitutes a reliable 20 method for determining the stateo-f crystallization of the metallic oxides to be treated.

It is also possible to determine the quantity expressed in percentage ofthe best suitable oxides in the following manner: 25

The colored oxide is filtered off from the suspension and the filtrateis placed in a colorimeter in which the filtrate is compared with anuntreated pigment solution of the same initial concentration. 3

The difference in color of the two solutions is proportional to thepercentage of the oxide examined with respect to the best suitableoxide, for instance, gamma corundum in the transition state, and may benumerically determined by 35 comparing it to a standard sample.

By mixing different oxides, it is possible to select the most suitablemixing proportion for the desired purpose so that reliable results forthe manufacture of shaped bodies made of non-plas- 40 tic metallicoxides are attained with certainty.

I claim as my invention:-

1. Method of examining the state of crystallization of calcined alumina,which comprises dissolving alizarin red in water, immersing thesubstance to be tested into the solution, stirring the solution so longas to cause a coloring of the sub stance to be tested, removing thesolid parts from the solution, drying them and testing microscopicallythe intensity of the coloring.

The material to be tested is immersed 5 the colored oxide from saidsolution by filtering and comparing the filtered solution with a testsolution of substantially the same initial pigment concentration as thefirst noted test solution to determine the pigment absorption of saidoxide. REINI-IOLD REICHMANN.

